I've always appreciated having someone (a skilled professor, a peer, or an upperclassman) point out interesting things in whatever I am studying. That way, it's easier to get "the point" of the subject or lesson. A lot of textbooks are designed to complement that aspect, rather than to provide it.

I've always thought that people teaching other people required a tight feedback loop and was difficult to do not in person. I'm curious, did you notice that kind of interaction going on at OpenStudy?